In general, a projection device such as a projection TV, a projection monitor, or the like is equipped with a rear projection screen projecting images toward the viewer's side. This rear projection screen is one of the optical display devices designed such that the images projected from the rear side of the screen pass through a viewing space. The viewing space may be relatively large (for example, a projection TV), or may be relatively small (for example, a projection monitor). The performance of a rear projection screen can be described by various characteristics of the screen. The screen performance typically includes gain, viewing angle, resolution, contrast ratio, color, and undesirable artifacts such as specks. The rear projection screen needs to have a high resolution, a high contrast ratio, and a high gain.
In addition, it is preferable that the rear projection television has a wide viewing angle capable of covering all the viewers within a broad range of angle. In order to achieve this wide viewing angle, the screen is provided with waveguides thereinside. In a rear projection screen, inherently, a point light source is positioned rearwards of the screen center and thus the incident angles of the light are different in the central area and the peripheral area of the screen. If the entire screen is formed of waveguides of the same structure, the reflection angles inside the waveguides are different from one section of the screen to another, due to their different incident angles. Some waveguides may not experience total reflection, depending on the position thereof. Usually, the waveguides, specially designed for a wide viewing angle, come to be placed in the central area of the screen. Therefore, in a case where the screen is designed with identically structured waveguides, the intended wide viewing angle can be achieved on the whole since the peripheral area of the screen is provided with the same waveguides. However, the luminance in the peripheral area is considerably degraded relative to the central area of the screen. For this reason, the image distinctiveness and clarity are made different in the central and peripheral areas of the screen, and homogeneity in the image is consequently degraded, thereby failing to achieve a high quality image.
In particular, a large-scale display device employs a plurality of unit light sources or a single diffusive light source, and the light intensity thus becomes non-uniform throughout the screen. More specifically, in the case where a plurality of light sources is used, the light intensity is lowered in the boundary area between the light sources. When a single diffusion light source is employed, the central and peripheral areas of the screen exhibit different luminance, due to different incident angles and different light-paths. For the above reasons, the brightness is not uniform over the entire screen, consequently resulting in non-uniform image distinctiveness and clarity, and degraded image resolution.